1. Field of the Invention
The present invention relates to a method and system for measuring vital signs, and particularly blood pressure, from a patient.
2. Description of the Related Art
Pulse oximeters are medical devices featuring an optical module, typically worn on a patient's finger or ear lobe, and a processing module that analyzes data generated by the optical module. The optical module typically includes first and second light sources (e.g., light-emitting diodes, or LEDs) that transmit optical radiation at, respectively, red (λ˜600-700 nm) and infrared (λ˜800-1200 nm) wavelengths. The optical module also features a photodetector that detects transmitted radiation that passes through an underlying artery within, e.g., the patient's finger or earlobe. Typically the red and infrared LEDs sequentially emit radiation that is partially absorbed by blood flowing in the artery. The photodetector is synchronized with the LEDs to detect the transmitted radiation. In response, the photodetector generates a separate radiation-induced signal corresponding to each wavelength. The signal, called a plethysmograph, varies in a time-dependent manner as each heartbeat varies the volume of arterial blood and hence the amount of radiation absorbed along the path of light between the LEDs and the photodetector. A microprocessor in the pulse oximeter digitizes and processes plethysmographs generated by the red and infrared radiation to determine the degree of oxygen saturation in the patient's blood using algorithms known in the art. A number between 94%-100% is considered normal, while a number below 85% typically indicates the patient requires hospitalization. In addition, the microprocessor analyzes time-dependent features in the plethysmograph to determine the patient's heart rate.
Another medical device called an electrocardiograph features conductive electrodes, placed at various locations on a patient's body, that measure electrical signals which pass into an amplification circuit. The circuit generates a waveform called an electrocardiogram, or ECG, that describes a time-dependent response of the patient's cardiovascular system.
Various methods have been disclosed for using both plethysmographs and ECGs, taken alone or in combination, to measure arterial blood pressure. One such method is disclosed in U.S. Pat. No. 5,140,990 to Jones et al. The '990 Patent discloses using a pulse oximeter with a calibrated auxiliary blood pressure measurement to generate a constant that is specific to a patient's blood pressure.
Another method for using a pulse oximeter to measure blood pressure is disclosed in U.S. Pat. No. 6,616,613 to Goodman. The '613 Patent discloses processing a pulse oximetry signal in combination with information from a calibrating device to determine a patient's blood pressure.
U.S. Pat. Nos. 5,857,795 and 5,865,755 to Golub each discloses a method and device for measuring blood pressure that processes a time difference between points on an optical plethysmograph and an ECG along with a calibration signal.
U.S. Pat. No. 6,511,436 to Asmar discloses a device for evaluating arterial wall stiffness by using pulse wave velocity measurements. The device estimates blood pressure using pulse wave velocity and a patient's biometric parameters.
Chen et al, U.S. Pat. No. 6,599,251, discloses a system and method for monitoring blood pressure by detecting plethysmographs at two different locations on a subject's body, preferably on the subject's finger and earlobe. The plethysmographs are detected using conventional pulse oximetry devices and then processed to determine blood pressure.
Inukai et al., U.S. Pat. No. 5,921,936, discloses a system that uses an electrocardiogram to detect the start of a heart beat and uses a cuff equipped with a pressure sensor to detect pulse waves in order to calculate a pulse transit time.
Suda et al., U.S. Pat. No. 5,788,634, describes a multi-purpose, clip-on sensor featuring a ‘gripper’ that includes an electrode pair and an optical system operating in a transmission mode. The electrode pair and optical system generate information that is processed outside of the sensor to make a blood pressure measurement.
Baruch et al., U.S. Pat. No. 6,723,054, describes an arm-worn system featuring two optical systems that measure two independent signals from a patient's arm. A processor calculates mathematical derivatives of the signals to derive a pulse transit time which can be used to calculate blood pressure.
Suga et al., U.S. Pat. No. 5,316,008, describes a wrist watch that features both optical and electrical sensors for measuring signals from a patient. During operation, the patient wears the wrist watch on one wrist, and places fingers from an opposing hand on the optical and electrical sensors. A pulse transit time is extracted from the signals and then used to calculate a blood pressure.